In this paper we report a study of novel thermally stable surface enhanced Raman scattering (SERS) substrates consisting of Ag nanoparticles immobilized on an anodized aluminum oxide (AAO) support.The morphological and chemical characteristics and the SERS activity of the Ag nanoparticles before and after the thermal treatment were evaluated using SEM, XPS, UV-vis and Raman spectroscopy. Our results show that the nanoporous surface of AAO significantly hinders the fusion of Ag nanoparticles to single large particles at up to 400 uC, preserving high SERS enhancement. XPS and SERS results indicate that exposure to high temperatures efficiently 'cleanses' the surface from citrate remnants, opening more binding sites for analytes on Ag nanoparticles. In addition, thermal decomposition of silver oxide occurs at 400 uC, ensuring a pure metallic surface and further enhancing SERS activity.

An unclad, multi-mode single crystal sapphire fiber was used as a platform, and immobilized colloidal Ag nanoparticles (NPs) were used as enabler, for evanescent-field fiber-optic sensing via surface-enhanced Raman scattering (SERS) of Rhodamine 6G (R6G) solution. The dependence of the measured Raman intensity on NP coverage density (to a maximum of 120  particles/μm²) as well as the coverage length (to a maximum of 6 cm) was investigated. We demonstrate the utility of SERS-active sapphire fibers for sensitive measurements (10⁻⁸ M R6G). We further reveal, with the aid of theoretical analysis, that multi-mode fiber offers a significant advantage compared to its single-mode counterpart because the former allows two orders of magnitude higher particle coverage density than the latter to maximize SERS benefit, while maintaining the dominance of Raman gain despite the competitive interplay of NP-induced absorption and scattering loss along the interaction path length.

Nanoscale pore channels of anodized aluminum oxide to endow entrapped silver nanoparticles within with structural and oxidation stability for potential surface-enhanced Raman scattering at elevated temperatures.

We here report an innovative and scalable strategy of transforming a commercial unclad sapphire optical fiber to an all-alumina nanostructured sapphire optical fiber (NSOF). The strategy entails fiber coating with metal aluminum followed by anodization to form alumina cladding of highly organized pore channel structure. Through experiments and numerical simulation, we demonstrate the utility and benefit of NSOF, analogous to all-silica microstructured optical fiber, for evanescent-field surface-enhanced Raman scattering (SERS) measurements. We experimentally reveal the feasibility of Ag nanoparticles (NPs)-enabled NSOF SERS sensing of 10−6 M Rhodamine 6G (R6G) after thermal treatment at 500 °C for 6 h by taking advantage of porous anodic aluminum oxide (AAO) structure to stabilize the Ag NPs. We show, via numerical simulations, that AAO cladding significantly increases the evanescent-field overlap, lower porosity of AAO results in higher evanescent-field overlap, and optimized AAO nanostructure yields greater SERS enhancement.